This proposal addresses a vital question of increasing relevance for the US HIV/AIDS epidemic: How does HIV Nef cause neuropathology and learning impairment? The success of combined antiretroviral therapy (cART) has greatly increased the life expectancy after infection by preventing progression to AIDS. This advance has resulted in greater emphasis on managing remaining morbidities. Even as clinical management of HIV replication has resulted in control of viral replication to below detectable levels, HIV associated neurocognitive disorders (HAND) appear with increasing prevalence. Several lines of evidence point toward production of HIV neurotoxins, rather than viral replication, as the cause of this progression. We developed a model of Nef neuropathology in rats that mimics the neuropathology that remains in the setting of viral control by cART. Nef is a known HIV neurotoxin that is produced by astrocytes even in the absence of viral replication. We found that rats infused into the hippocampus with autologous astrocytes modified to produce Nef showed inflammation, neuronal loss, and learning impairment. Our preliminary studies led us to propose a hypothesis that Nef drives inflammation leading to neuronal apoptosis and learning impairment. Our data suggest roles for TGF signaling and CCL2 in this pathology. We will address the hypothesis in two aims. The first will focus on identifying the mechanism and extent of neuron loss in relation to the degree of learning impairment in our model. A major focus of the studies in this aim is to identify the cause and effect relationships that produce the outcomes in the model we have already identified. The second aim will investigate the mechanistic role for TGF signaling in the inflammation and neurotoxicity caused by astrocyte expression of Nef. This aim will be accomplished through a series of in vitro experiments with primary cells and cell lines to probe the requirement of TGF signaling in Nef neurotoxicity. The outcomes of this proposal will move us forward in understanding the mechanism of Nef neurotoxicity and learning impairment. Completion of these studies is expected to identify biological targets for development of future treatments to address the next frontier in HIV clinical management. Now that suppression of replication has changed HIV infection to a chronic illness rather than a sentence of certain death, the changing nature and duration of morbidities like HAND require novel therapies where none currently exist.